It has long been recognized in the anesthetic and other health-care related arts that monitoring respiration is a very reliable method of determining whether a patient is still alive, and moreover one which provides an instant indication of trouble, as compared with other life signs which take longer periods to depart from normal values. Such an indication is of great interest with patients undergoing surgery or in other life-threatening situations, and in connection with infants who, as is well-known, are subject to cessation of breathing (apnea) for no apparent cause. Therefore, it is desirable that means be provided for monitoring the continued breathing of an individual.
One method of monitoring breathing which has been extensively employed in the prior art involves monitoring the difference in carbon dioxide (CO.sub.2) content between an individual's inspired and expired gas streams. It is impossible to breathe without the CO.sub.2 content of the expired gas stream varying from that of the inspired gas stream by at least about 2%.
The prior art has recognized the above, yet applicant is aware of no system in the prior art which simply monitors the relative CO.sub.2 content of the inspired stream and that of the expired stream and uses these to provide an indication of proper breathing or not, as the case may be. Instead, the prior art, as exemplified by the Hewlett-Packard Model 47210A capnometer, has tended toward systems of ever-increasing complexity and cost, providing in many cases values for the absolute amount of CO.sub.2 present in the expired stream, which is unnecessary for a life monitor. Applicant has realized that all that needs to be detected is the relative difference in the CO.sub.2 contents of the inspired and expired streams.
It seems, therefore, that a need exists in the art for a simplified and improved respiration monitor of reduced complexity and cost, which operates by simply comparing the relative CO.sub.2 contents of the inspired and expired gas streams, and to provide such is accordingly an object of the invention.
The prior art has generally monitored the amount of CO.sub.2 in a gas stream using infrared absorption measurement techniques. According to this technique, an infrared light source radiates through a cuvette having spaced parallel windows in which is enclosed a sample of the gas to be monitored. The radiation then falls on a suitable detector, typically a Golay cell such as Beckman Instruments' Model LB-2, which is highly sensitive to acoustic noise, or a PbSe photodetector, which is very sensitive to low-frequency noise, a significant defect in the environment of this invention.
It is an object of this invention to avoid use of these and other non-optimal detectors.
According to one aspect of the invention, radiation is monitored by a thermopile (a combination of a number of individual thermistors, so as to be very sensitive to small temperature changes). Such thermopiles are useful even at low frequencies and in connection with D.C., unlike the other detector types mentioned.
As in the prior art, the detector output is monitored. As infrared radiation of particular wavelengths is preferentially absorbed by carbon dioxide, the output of a detector sensitive to those wavelengths provides an inverse indication of the relative amount of CO.sub.2 in the gas stream. By calibration using a sample of known concentration, or possibly by other methods, the detector can be made to yield quite accurate results; however, as mentioned above, the object of the invention is not so much to provide intrinsically accurate results concerning the actual amount of CO.sub.2 present in a patient's exhaled gas stream, but merely to determine whether the patient is breathing. Therefore, according to the invention, calibration is ordinarily not performed, and the instrument is simply operated so as to detect variation in the CO.sub.2 contents of the inspired and expired gas streams.
Accordingly, it is an object of the invention to provide a respiration detector which operates by detecting the differences between the CO.sub.2 concentration of inspired and expired gas streams, by providing a source of infrared radiation disposed in juxtaposition to a cuvette containing the gas stream to be measured, and by monitoring the output of a thermopile disposed opposite the infrared source on the other side of the cuvette, which does not require calibration for proper operation.
It will be appreciated by those skilled in the art that measurement of the CO.sub.2 content of a sample by radiation adsorption techniques is subject to error due to a number of factors. Though as mentioned above, the invention of the applicants is not so much concerned with measurement of the absolute value of the CO.sub.2 in the patient's breath as between the relative difference in CO.sub.2 content between the inspired and expired streams, it is nevertheless desirable that the instrument be operated in the optimal fashion and furthermore that it be enabled to operate under less than ideal conditions which may include, for example, clouding of the windows through which the infrared beam passes due to humidity, temperature drift and the like. Further, it is desirable that the instrument measure the relative difference in the CO.sub.2 contents of the streams accurately regardless of their absolute value, i.e. whether the difference is between 0 and 3% CO.sub.2 or between 6 and 9%. Finally, it is desirable that the absolute amount of radiation incident on the thermopile be substantially constant so that it can be well matched to the input value which provides the greatest dynamic range in the thermistor output signal.
For all of the above reasons, it is desirable that means be provided to regulate the intensity of the radiation falling on the thermopile, so as to compensate the system for all the sources of possible error discussed above.
In the preferred embodiment, this is achieved by provision of a feedback loop connecting the output of the thermopile with the power supply which is used to operate the source of infrared radiation. Use of feedback in a loop having a low pass filter therein ensures that variation in the inspired and expired gas stream will not cause the bulb output to vary, while ensuring that the mean output of the thermopile is maintained at a substantially constant level, thus compensating for all the sources of error just discussed.
The selection of the infrared source has also been a source of some difficulty and expense in the prior art. The applicant has realized that, by using certain ordinary incandescent lamp bulbs, an adequate amount of infrared light of suitable wavelength is provided to enable suitable detection, thus further simplifying and reducing the cost of the apparatus of the invention.
It is therefore an object of the invention to further simplify and reduce the cost of the apparatus of the invention by using an inexpensive source of infrared radiation.
The prior art has also used costly sapphire windows for the windows of the cuvette. It is an object of the invention to avoid use of such materials.
It will be appreciated by those skilled in the art that the output of typical thermopiles is a relatively small voltage, usually on the order of millivolts. It will further be appreciated by those skilled in the art that in the typical hospital operating room environment of today, there are frequently a large number of electronic devices, some of which may not be properly shielded. Accordingly, it is desirable that the instrument be designed in such a way as to be less sensitive to electromagnetic noise than otherwise, and furthermore that the signal processing circuitry to which the thermopile is connected be enabled to differentiate between noise and signal insofar as reasonably possible, given the goals of simplicity and low cost, as discussed above, and such is accordingly an additional object of the invention.
It is furthermore desirable that the gas stream be monitored as closely to the patient as possible, to obtain accurate results. Accordingly, the sensor of the respiration monitor of the invention should be lightweight, so as to be attachable directly to flexible tubing connected closely to the patient while still achieving the other objects of the invention discussed above, and such is accordingly an additional object of the invention.